Increased caloric intake in dietary obesity could be driven by central mechanisms that regulate reward-seeking behavior. The mesolimbic dopamine system, and the nucleus accumbens in particular, underlies both food and drug reward. We investigated whether rat dietary obesity is linked to changes in dopaminergic neurotransmission in that region. Sprague-Dawley rats were placed on a cafeteria-style diet to induce obesity or a laboratory chow diet to maintain normal weight gain. Extracellular dopamine levels were measured by in vivo microdialysis. Electrically evoked dopamine release was measured ex vivo in coronal slices of the nucleus accumbens and the dorsal striatum using real-time carbon fiber amperometry. Over 15 weeks, cafeteria-diet fed rats became obese (>20% increase in body weight) and exhibited lower extracellular accumbens dopamine levels than normal weight rats (0.007±0.001 vs. 0.023±0.002 pmol/sample; P<0.05). Dopamine release in the nucleus accumbens of obese rats was stimulated by a cafeteria-diet challenge, but it remained unresponsive to a laboratory chow meal. Administration of d-amphetamine (1.5 mg/kg i.p.) also revealed an attenuated dopamine response in obese rats. Experiments measuring electrically evoked dopamine signal ex vivo in nucleus accumbens slices showed a much weaker response in obese animals (12 vs. 25 × 10 6 dopamine molecules per stimulation, P<0.05). The results demonstrate that deficits in mesolimbic dopamine neurotransmission are linked to dietary obesity. Depressed dopamine release may lead obese animals to compensate by eating palatable "comfort" food, a stimulus that released dopamine when laboratory chow failed.Keywords nucleus accumbens; striatum; feeding; body weight; amphetamine; hyperphagia The rapid rise of dietary obesity in industrialized societies indicates that non-homeostatic signaling pathways that allow for chronic positive energy intake may be responsible. A crucial question is why laboratory animals and humans keep on eating energy-rich, palatable food to the degree that they become obese. From an evolutionary perspective, it is to be expected that the brain developed a system to respond to natural rewards, such as food. These central mechanisms are conserved across species in order to ensure survival (Kelley and Berridge,

Among the aminopyridines, 3,4-diaminopyridine (DAP) is a more effective K+ channel blocker than is 4-aminopyridine (4-AP), and, furthermore, DAP enhances neuromuscular transmission. Because 4-AP improves muscle contractility, we hypothesized that DAP would also increase force and, in addition, ameliorate fatigue and improve the neurotransmission failure component of fatigue. Rat diaphragm strips were studied in vitro (37 degrees C). In field-stimulated muscle, 0.3 mM DAP significantly increased diaphragm twitch force, prolonged contraction time, and shifted the force-frequency relationship to the left without-altering peak tetanic force, resulting in increased force at stimulation frequencies < or = 50 Hz. During 20-Hz intermittent stimulation, DAP increased diaphragm peak force compared with control during a 150-s fatigue run and, furthermore, significantly improved maintenance of intratrain force. The relative contribution of neurotransmission failure to fatigue was estimated by comparing the force generated by phrenic nerve-stimulated muscles with that generated by curare-treated field-stimulated muscles. DAP significantly increased force in nerve-stimulated muscles and, in addition, reduced the neurotransmission failure contribution to diaphragm fatigue. Thus DAP increases muscle force at low-to-intermediate stimulation frequencies, improves overall force and intratrain fatigue during 20-Hz intermittent stimulation, and reduces neurotransmission failure.

The mechanism of the nearly universal decreased muscle strength in cirrhosis is not known. We evaluated whether hyperammonemia in cirrhosis causes contractile dysfunction independent of reduced skeletal muscle mass. Maximum grip strength and muscle fatigue response were determined in cirrhotic patients and controls. Blood and muscle ammonia concentrations and grip strength normalized to lean body mass were measured in the portacaval anastomosis (PCA) and sham-operated pair-fed control rats (n = 5 each). Ex vivo contractile studies in the soleus muscle from a separate group of Sprague-Dawley rats (n = 7) were performed. Skeletal muscle force of contraction, rate of force development, and rate of relaxation were measured. Muscles were also subjected to a series of pulse trains at a range of stimulation frequencies from 20 to 110 Hz. Cirrhotic patients had lower maximum grip strength and greater muscle fatigue than control subjects. PCA rats had a 52.7 ± 13% lower normalized grip strength compared with control rats, and grip strength correlated with the blood and muscle ammonia concentrations (r(2) = 0.82). In ex vivo muscle preparations following a single pulse, the maximal force, rate of force development, and rate of relaxation were 12.1 ± 3.5 g vs. 6.2 ± 2.1 g; 398.2 ± 100.4 g/s vs. 163.8 ± 97.4 g/s; -101.2 ± 22.2 g/s vs. -33.6 ± 22.3 g/s in ammonia-treated compared with control muscle preparation, respectively (P < 0.001 for all comparisons). Tetanic force, rate of force development, and rate of relaxation were depressed across a range of stimulation from 20 to 110 Hz. These data provide the first direct evidence that hyperammonemia impairs skeletal muscle strength and increased muscle fatigue and identifies a potential therapeutic target in cirrhotic patients.

van Lunteren E, Moyer M. Oxidoreductase, morphogenesis, extracellular matrix, and calcium ion-binding gene expression in streptozotocin-induced diabetic rat heart.

The aminopyridines block several types of potassium (K+) channels and exert a direct inotropic effect on skeletal muscle by prolonging the duration of the action potential. Aging influences skeletal muscle Cl- channels and their regulation, and affects both resting whole-cell K+ conductance and adenosine triphosphate (ATP)-sensitive K+ channels, although in opposite directions. The present study tested the hypothesis that aging affects diaphragm-muscle K+ channels responsible for repolarization of the action potential and force production. Diaphragms of young adult (age 3 to 4 mo) and old (age 20 to 21 mo) male Fischer 344 rats was studied in vitro at 37 degrees C. The K+-channel blocker 3,4-diaminopyridine (DAP, 0.3 mM) did not alter resting membrane potential or action-potential height, overshoot, or rate of depolarization of either young-adult or old muscle. However, DAP slowed the rate of repolarization of the action potential and increased the action-potential area in young-adult and old muscle; the time for the action potential to repolarize by 80% increased from 0.59 +/- 0.02 ms (mean +/- SE) to 3.37 +/- 0.68 ms (p < 0.05) in young-adult muscle and from 0.87 +/- 0.06 ms to 2.52 +/- 0.54 ms (p < 0.05) in old muscle, whereas the action-potential area increased from 56 +/- 3 mVms to 193 +/- 34 mVms (p < 0.05) in young-adult muscle and from 72 +/- 5 mVms to 134 +/- 20 mVms (p < 0. 05) in old muscle. The action-potential area was not different in young-adult and old diaphragm without DAP, but was significantly larger in young-adult than in old diaphragm with DAP (p < 0.05). The functional consequence was that DAP increased diaphragm isometric twitch force by 181 +/- 12% (p < 0.05) in young-adult muscle and by 144 +/- 24% (p < 0.05) in old muscle; the increase was significantly greater in young-adult than in old muscle (p < 0.05). These data suggest an aging-associated reduction in, or reduced DAP sensitivity of, diaphragm K+ conductance during action potentials, which most likely reflects aging-associated alterations in delayed-rectifier K+ conductance. Although the inotropic effect of DAP was greater for young-adult than for old diaphragm muscle, the difference was sufficiently modest to show that DAP has substantial inotropic effects in old muscle.

Neuromuscular junction endplate potentials (EPPs) decrease quickly and to a large extent during continuous stimulation. The present study examined the hypothesis that EPP rundown recovers rapidly, thereby substantially preserving neurotransmission during intermittent compared with continuous stimulation. Studies were performed in vitro on rat diaphragm, using mu-conotoxin to allow recording of normal-sized EPPs from intact fibers. During continuous 5- to 100-Hz stimulation, EPP amplitude declined with a biphasic time course. The initial fast rate of decline was modulated substantially by stimulation frequency, whereas the subsequent slow rate of decline was relatively frequency independent. During intermittent 5- to 100-Hz stimulation (duty cycle 0.33), EPP amplitude declined rapidly during each train, but recovered substantially by the onset of the following train. The intra-train declines were substantially greater than the inter-train declines in EPP amplitude. Intra-train reductions in EPP amplitude were stimulation frequency dependent, based on both the total decline and rate constant of EPP decline. In contrast, the degree of recovery from train to train was independent of stimulation frequency, indicating low frequency dependence of inter-train rundown. The substantial recovery of EPP amplitude in between trains resulted in greater cumulative EPP size during intermittent compared with continuous stimulation. During continuous stimulation, EPP drop-out was only seen during 100-Hz stimulation; this was completed mitigated during intermittent stimulation. Miniature EPP size was unaffected by either continuous or intermittent stimulation. The pattern of rapid intra-train rundown and slow inter-train rundown of EPP size during intermittent stimulation is therefore due to rapid changes in the magnitude of neurotransmitter release rather than to axonal block or postsynaptic receptor desensitization. These findings indicate considerable rundown of EPP amplitudes within a stimulus train, with near complete recovery by the onset of the next train. This substantially attenuates the decrement in EPP amplitude during intermittent compared with continuous stimulation, thereby preserving the integrity of neurotransmission during phasic activation.

Despite the promising potential of microfluidic artificial lungs, current designs suffer from short functional lifetimes due to surface chemistry and blood flow patterns that act to reduce hemocompatibility. Here, we present the first microfluidic artificial lung featuring a hemocompatible surface coating and a biomimetic blood path. The polyethylene-glycol (PEG) coated microfluidic lung exhibited a significantly improved in vitro lifetime compared to uncoated controls as well as consistent and significantly improved gas exchange over the entire testing period. Enabled by our hemocompatible PEG coating, we additionally describe the first extended (3 h) in vivo demonstration of a microfluidic artificial lung.

K+ channel blockers increase skeletal muscle force during twitch contractions; the present study determined whether K+ channel blockade also modulates force during longer term and higher frequency stimulation. 4-Aminopyridine (4-AP; 0.3 mM) increased rat diaphragm force during twitch, 5-Hz and 20-Hz but not 100-Hz stimulation, and prolonged isometric contraction but not half-relaxation time. In response to continuous 5-Hz stimulation, the rate of force decline was accelerated by 4-AP so that over time force dropped below that of control muscle strips. In response to intermittent 20-Hz stimulation, 4-AP produced an early force potentiation; the 4-AP-induced force increase was maintained throughout repetitive stimulation despite an accelerated rate of force decline. In response to continuous 100-Hz stimulation, 4-AP did not affect rate of force decline. During 5- and 20-Hz stimulation, there was an interaction between 4-AP and duration of stimulation in prolonging contraction and especially half-relaxation time. Tetraethylammonium (10 mM) augmented diaphragm force less than did 4-AP, did not affect rate of force decline during 5-Hz stimulation, and did not interact with fatigue to prolong isometric twitch kinetics. These data indicate that K+ channel blockade with 4-AP increases diaphragm force at low to intermediate stimulation frequencies, may increase early force potentiation during repetitive contraction, and depending on stimulation pattern either accelerates or has no effect on rate of fatigue.